1. Field of the Invention
The present invention relates to monitoring the rotational speed of the output shaft of an electric motor and, particularly, to the provision of an indication that the load driven by a three-phase machine has reached a stand-still state subsequent to interruption of the current supply to the machine. More specifically, this invention is directed to a monitor which electronically determines when the rotor of a three-phase electrical motor, which has been disconnected from its power supply, has reached a stand-still state. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well-suited for employment with machine tools. The moving parts of such tools are frequently driven by three-phase electrical motors. These moving parts, which may rotate, reciprocate and/or comprise components of robotic production lines, constitute hazards to personnel when in operation. Accordingly, machine tools or the workpiece manipulating components thereof are frequently located in an area which is protected by guards or the like, i.e., access to the moving parts is via protective doors. Such doors, or other safety related devices, are controlled in such a manner that they cannot be opened and/or cannot be operated until the associated machine has come to rest. As a consequence of inertia and kinetic energy, the rest condition does not occur until some time after the electrical power supply to the drive motor has been interrupted.
An electric motor will, upon being disconnected from its power source, function as a generator during the time the rotor slows down from the operating speed to a stand-still condition. The zero-speed monitoring of the output shaft of electric motors has conventionally been accomplished by comparing the voltage generated by the machines with a reference voltage level, the machine being assumed to be at rest if the latter is undershot. This reference voltage level, however, must be set and it is, accordingly, possible that either the setting will be incorrect or will accidentally be changed. If the reference voltage level is incorrect, there is a potential safety hazard.